Vanessa is 4 times as old as Tiffany and is also 21 years older than Tiffany. How old is Vanessa?
Answer: We can use the given information to write down two equations that describe the ages of Vanessa and Tiffany. Let Vanessa's current age be $v$ and Tiffany's current age be $t$ $v = 4t$ $v = t + 21$ Now we have two independent equations, and we can solve for our two unknowns. One way to solve for $v$ is to solve the second equation for $t$ and substitute that value into the first equation. Solving our second equation for $t$ , we get: $t = v - 21$ . Substituting this into our first equation, we get the equation: $v = 4$ $(v - 21)$ which combines the information about $v$ from both of our original equations. Simplifying the right side of this equation, we get: $v = 4v - 84$ Solving for $v$ , we get: $3 v = 84$ $v = 28$.